“Because they have identical
DNA, it ought to be 100%,” Dr. Whitehead notes. But the studies reveal something
else. “If an identical twin has same-sex attraction the chances the co-twin has
it are only about 11% for men and 14% for women.”

"Because identical twins are
always genetically identical, homosexuality cannot be genetically dictated.
“No-one is born gay,” he notes. “The predominant things that create
homosexuality in one identical twin and
not in the other have to be post-birth factors.” ("Identical Twin Studies Prove
Homosexuality is Not Genetic", June 24, 2013, orthodoxy.net Blog)